When elementary OS 'Loki' is coming?

After release of Ubuntu 16.04 LTS, there are a lot of queries regarding release of elementary OS 'Loki', 4th major release of elementary OS. Elementary OS is a Ubuntu LTS based GNU/Linux distribution that combines stable Ubuntu core with a custom made desktop environment, Pantheon Desktop, which resembles OS X.

Normal ubuntu releases and it's derivatives makes major updates with each stable releases and then provide minor updates during it's life cycle if it is necessary. Elementary OS works in a different way. They don’t hold back updated packages unless it’s necessary on a technical level

A blog post published by elementary OS team explains their plans for elementary OS 0.4 'Loki'.

With the recent release of Ubuntu 16.04, the inevitable flood of “When
is Loki coming?” posts have begun pouring in. So here we are to help
answer the question that some of you know the answer to already, “When
it’s ready ;)”. But what exactly does that mean? How do we (and you)
know when the 4th major release of elementary OS, code named Loki, will
be ready to use?

If this is your first time following along with elementary OS
development then it’s very important for us to note that we work a
little bit differently. Instead of basing our releases on how much time
has passed, we base them on how much work we can get done. We want every
new release of elementary OS to be significantly better than the last
release. We don’t hold back updates from previous releases unless it’s
necessary on a technical level. That’s why our last release, Freya, had
several updates throughout the past year. Major releases happen when
large technical changes happen. For developers, you’ll know these
changes as “API breaks”.